The main goals of the proposed research are to characterize the neuroendocrine substrates involved in generation of circannual rhythms in behavior and physiology, and to identify environmental stimuli that synchronize these endogenous rhythms. The model species to be studied is the golden-mantled ground squirrel. Specific projects include: 1) A definitive test of the role of day length in entrainment of circannual rhythms, 2) establishing the role of the retinohyothalamic tract in photic entrainment of circannual rhythms, 3) determining if the lateral septal nuclei, medial pre-optic region and mammillary nuclei are part of the neural substrate for generation and expression of circannual rhythms, 4) establishing whether the period of circadian rhythms is temperature-dependent during part of the circannual cycle, 5) evaluating interactions among body fat, energetics and reproduction during the circannual cycle, and, 6) establishing the sequence of lipid mobilization from different fat depots during the weight loss phase of the circannual cycle, as well as differences between juveniles and adults in number and size of adipocytes. Parameters to be measured include body mass, adipocyte number and size, plasma testosterone levels, circadian activity rhythms, and external reproductive condition. The proposed work is relevant to mechanisms that underlie endogenous seasonal affective disorders in human beings, and could ultimately lead to pharmacological as well as non-invasive treatments for these conditions.